See, to be honest, 100 marks in JEE Main is not considered to be a great score as this score will hardly provide you with some good college opportunities. And on the other hand, 300+ in BITSAT is considered to be a splendid score and will open many doors for you in top branches in all the campuses of BITS Pilani.

If you have not done JEE Main exam with the capability that you have, then there won't be a problem as BITSAT is usually of lower level than JEE Main. But if not, then you'll have to give your best efforts in preparing for JEE Main.

Anyways, no one can stop you if you give your best efforts in preparing for BITSAT in the time that you get between JEE Main and BITSAT. So, keep preparing.

Hope I have answered your question. All the best!